Ancillary services are the things you would need to complete your holiday, apart from booking your hotel and accommodation. The ancillary's is what makes the holiday and are all the extra important pieces you would need to sort out.
For example with travel insurance if your luggage is lost at the airport the travel insurance would cover that and help and secure you along the way of finding your luggage no matter what it took.
Car Hire/Insurance/Airport Parking all come under the same category, you would need to have a valid driver's license to drive over in a different country, you would have to pay for the insurance on that car to use it abroad and pay for the parking at the airport.
Again if you are visiting different parks, in Florida maybe you would need to have a car to travel to all the different parks or events that might be happening on your holiday, therefore you would need to purchase your park/event tickets. Before you even go on holiday if it's an outbound holiday you would have to change your currency for example if you was going on holiday to Spain you would have to change pounds to euros or Florida; pounds to dollars. There are many different aspects to ancillary's which are very important and make up your holiday and make it secure and easier for you, if don't have many of these aspects covered especially if you know you need them to get through your holiday then you probably aren't going to get very far and if anything goes wrong.